1. Field of the Invention
The present invention generally relates to a system for determining a starting position angle for a rotor of a DC motor.
2. Description of Related Art
Actively controlled power electronic switches (such as MOSFETs', IGBTs', SCRs', etc.) can substantially improve the performance of some alternators and motors. MOSFETs' have been employed in a variety of rectifier designs for automotive alternator applications. When switches are used in an alternator rectifier, the firing of the switches can be manipulated in such a way that the alternator can function as a DC motor to start the vehicle engine. The electric machine used both as starter and alternator in a vehicle is called alternator/starter. It can be used to implement the stop/start feature of a vehicle and this feature can substantially improve the overall fuel efficiency of a vehicle.
When electric switches, such as MOSFETs', are used for controlling an electric machine, detection of the rotor angle position forms an integral part of the control algorithm. Current in each phase of the machine is controlled by the MOSFETs' based on rotor angle position. Traditionally, external rotor angle position sensors, such as, optical encoders, Hall-effect magnetic sensors, and resolvers, are used to directly detect the angular position of the rotor. Unfortunately, these sensors are often expensive, temperature sensitive, and difficult to package within an alternator. A sensorless control approach can provide increased power density while minimizing the cost of the machine.
Sensorless techniques typically monitor phase back EMF, phase terminal voltages, or phase currents in order to determine the rotor angle position. When the electric machine is not spinning, pre-existing sensorless techniques are not adequate for determining the rotor position.
In most DC motor applications utilizing sensorless techniques, the rotor is pre-positioned to a known rotor angle position by energizing particular windings of the electric machine. With the rotor in a known position, standard 6-step control process is implemented to drive the electric machine. If the rotor is pre-rotated to a known position, the pre-rotated angle could be clockwise, or counter-clockwise, depending on the initial rotor angle position.
When the alternator function as a vehicle starter (DC motor), the alternator rotor starts at an arbitrary angle. Because the alternator is connected to the engine crankshaft through a drive belt and since the engine can only rotate in one direction, the alternator must turn the drive belt in only one direction. For example, a clockwise direction assuming the engine normal operation direction is clockwise. The present sensorless techniques for pre-positioning the rotor cannot ensure a clockwise rotation to the known position.
In view of the above, it is apparent that there exists a need for an improved system for determining a starting position angle of an electric motor.